Field of the Invention
This invention relates to a method and apparatus for reducing defects in flat glass, and in particular to reducing tin defects in glass made by the float process. 2. Technical Considerations
In forming flat glass using the process known as the "float process", molten glass is delivered onto a pool of molten metal in a float forming chamber and attenuated to form a ribbon of desired thickness. Glass entering the float forming chamber is initially at a relatively high temperature, for example around 2000.degree. F. (1100.degree. C.), so as to be relatively fluid. During its passage through the float chamber, the glass is cooled to a condition suitable for engagement with attenuating devices, for example to a temperature around 1400-1700.degree. F. (760-930.degree. C.), and subsequently cooled further to a dimensionally stable condition so that it may be withdrawn from the forming chamber, typically to a temperature about 1100.degree. F. (600.degree. C.). The molten metal, usually molten tin or an alloy thereof, and the atmosphere within the forming chamber are generally at a temperature lower than the glass temperature and thus considerable cooling of the glass takes place naturally by losing heat to the surroundings in the forming chamber.
Processing the glass ribbon by the float process presents several problems which may result in tin defects on the upper and lower surfaces of the glass ribbon. More specifically, molten tin in the float forming chamber reacts readily with oxygen within the forming chamber atmosphere or dissolved in the molten tin, forming tin oxide which vaporizes, collects in the forming chamber atmosphere and condenses on cooler portions of the forming chamber roof structure. It is well known to use a protective atmosphere, generally a mixture of nitrogen and hydrogen, in the float forming chamber to prevent oxidation and contamination of the molten tin. Depending on the operating conditions within the forming chamber, the hydrogen will react with the condensed tin oxide to form water vapor and elemental tin, which, in turn, may drop from the roof structure onto the surface of the hot glass ribbon, forming a defect. In addition to tin oxide formation during the manufacture of flat glass by the float process, molten tin will react with sulfur in the tin and form tin sulfide which also vaporizes, collects within the forming chamber and condenses on cooler parts of the forming chamber's roof structure. The tin sulfide may also be reduced by hydrogen, producing hydrogen sulfide and elemental tin which, in turn, may drop from the roof onto the glass ribbon.
It would be advantageous to reduce the formation of tin oxide in the float forming chamber so as to reduce the number of tin related defects, as well as to keep hydrogen out of the forming chamber atmosphere to reduce defects resulting from the formation of tin sulfide. In addition, it is believed that the reduction of oxygen in the molten tin will reduce diffusion of ionic tin into the bottom surface of the glass ribbon along the glass/molten tin interface. This ionic tin diffusion may lead to bloom, which is the term used for surface wrinkling of flat glass when it is heated and shaped, and the formation of calcium carbonate deposits on the lower surface of the glass ribbon.